11.1.6 11.18: Mammals are animals that have hair and produce milk.

As we saw in the previous section, two different evolutionary lineages began to diverge after amniotic vertebrates appeared. One gave rise to the reptiles, and the other gave rise to the mammals. Originally small, nocturnal insect-eaters, the early mammals remained small. And, judging from the scarce fossil record, they were never in great abundance while the dinosaurs were around. Early mammals had several features in common that are still present in all mammals today. Two important features are hair—dead cells filled with the protein keratin—which serves as an insulator, and mammary glands in female mammals, which enable them to produce nutritious, calorie-rich milk and nurse their young.

As the earliest mammals evolved, there was a gradual transition from bodies with short legs that projected out horizontally from the trunk, like the legs of an alligator, to bodies with long legs held vertically beneath the trunk, like those of a dog (FIGURE 11-30). These anatomical changes would have allowed the earliest mammals to run faster and farther to capture prey. But an increase in muscle activity for running had to be accompanied by an increase in the rate of cellular respiration to provide the energy to keep the leg muscles working. With this increase in metabolism, along with the evolution of hair, the early mammals could trap the heat produced and use it for temperature regulation, setting the stage for endothermy (but through a different sequence of events than in the birds).

One feature common to most mammals is viviparity, or giving birth to babies (“live birth”) rather than laying eggs, but it is not a defining mammalian characteristic—it isn’t common to all mammals. Monotremes retain the ancestral condition of laying eggs. Monotremes do produce milk, but they do not have nipples—their mammary glands open directly onto the skin, so babies can lap up milk from the skin and hair.

Today, only five species of monotremes survive: the platypus and four species of spiny animals called echidnas. The platypus lives in streams and rivers in eastern Australia. It uses its broad, leathery, electrosensitive bill to probe under rocks and sunken logs for prey such as aquatic insect larvae and crayfish, which it finds by sensing the electrical activity of their muscles as they try to hide. The echidnas—three species in Australia and one in New Guinea—also have a leathery electrosensitive snout.

The remaining two lineages of mammals, the marsupials and the placentals, are viviparous, but the newborn young of marsupials and placentals are quite different (FIGURE 11-31). Marsupials are called the “pouched mammals” because the female of most marsupial species has a pouch on her abdomen in which the young complete their development, following a short period of embryonic life in the uterus.

Placental mammals (including humans) take their name from the placenta, which is the structure responsible for the transfer of nutrients, respiratory gases, and metabolic waste products between the mother and the developing fetus. Dense capillary beds in the placenta send finger-like projections into the wall of the uterus, where they are surrounded by blood from the mother’s circulation.